A person in California who was not exposed to anyone known to be infected with the coronavirus, and had not traveled to countries in which the virus is circulating, has tested positive for the infection.

It may be the first case of community spread in the United States, the Centers for Disease Control and Prevention said on Wednesday.

“At this point, the patient’s exposure is unknown,” the C.D.C. statement said. “The case was detected through the U.S. public health system and picked up by astute clinicians.”

The case was announced shortly after President Trump concluded a news briefing in which he said that aggressive public health containment measures and travel entry restrictions had successfully limited the spread of coronavirus in the United States.

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It brings the number of cases in the country to 60, including the 45 cases among Americans who were repatriated from Wuhan, China — the epicenter of the outbreak — and the Diamond Princess cruise ship, which was overwhelmed by the virus after it docked in Japan.

Until now, public health officials have been able to trace all of the infections in the country to a recent trip abroad or a known patient, and to identify the sources of exposure.

Though C.D.C. officials said it was possible the patient was exposed to a returning traveler who was infected, the new case appears to be one of community spread — one in which the source of infection is unknown.

“The thing that would immediately make all of us uneasy is if this person has no direct contact with someone who comes from an affected country,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University.

“That would suggest there are other undetected cases out there, and we have already started some low-grade transmission.”

Public health officials said the infected individual was a resident of Solano County, Calif., and was receiving medical care in Sacramento County, but they have not disclosed any other information to protect the patient’s privacy.

Doctors in the patient’s community may want to expand testing for the coronavirus among their patients in order to identify other infected individuals there, Dr. Schaffner said.

“Physicians are going to want to start testing everybody with a fever and a cough,” he said. “They’re going to want to do that and the patients are going to want to do that.”

Right now, however, testing capacity is limited in the United States.

The C.D.C. developed a diagnostic test and distributed testing kits to local health departments around the country.

But the kits were flawed, and the agency must manufacture new ones. Although a dozen states are capable of testing for coronavirus infection, confirmatory tests must still be done by the C.D.C., a process that can take days.

Please check back for updates to this developing story.